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If the capture of Mosul — a city of 2 million people — stands, the al-Qaida offshoot Islamic State of Iraq and the Levant would become the most significant jihadist organization in the world, eclipsing core al-Qaida.

Militants stormed a university filled with hundreds of students in Iraq's restive Anbar province Saturday, briefly taking students hostage before withdrawing from the school amid gunfire, officials and witnesses said. Meanwhile, fighting in a northern city killed 21 police officers and 38 militants, authorities said.

With insurgents linked to al-Qaida battling for control of two major Iraqi cities, long-standing suspicion between the Obama administration and the government in Baghdad is hindering joint efforts against a common foe.

WASHINGTON — The American military lost at least 200 troops in Mosul, Iraq’s second largest city, during the American occupation of Iraq, almost all of them to attacks by what today is known as the Islamic State of Iraq and the Levant.

But there was little the United States could do Tuesday as hundreds of insurgents from that same organization swept through Mosul, quickly capturing the airport, three military bases, the central bank vault and a prison, where it released 3,000 detainees, most of whom were ISIL members swept up in earlier counterterrorism campaigns.

U.S. officials were quick to express solidarity with the Iraqi government of Prime Minister Nouri al-Maliki, who was elected to his post originally during the American occupation and whose administration the U.S. has backed with weapons shipments and military training. State Department spokeswoman Jen Psaki said the United States was working closely with al-Maliki’s government, and Brett McGurk, the State Department’s top diplomat for Iraq and Iran, pointed out via Twitter that U.S. and Iraqi soldiers “have suffered and bled together, and we will help in time of crisis.”

But the nature of that help was perhaps best encapsulated in the response of Pentagon spokesman Navy Rear Adm. John Kirby that made it clear that the U.S. was unlikely to become directly involved in Iraq’s battle with ISIL. “This is for the Iraqi security forces and the Iraqi government to deal with,” Kirby said.

In comments Tuesday, U.S. officials left no room for direct involvement in the conflict there, where ISIL, analysts said, had demonstrated that it could successfully and simultaneously control parts of two major Iraqi cities, while battling multiple forces inside Syria, including the Lebanese militant organization Hezbollah and al-Qaida’s Nusra Front.

Worst-case scenarios were common as observers wondered whether ISIL, also known as the Islamic State of Iraq and Syria, or ISIS, could maintain its control of Mosul and Fallujah, the scene of two fierce Marine Corps offensives against ISIL a decade ago, while trying to push into places such as Tikrit and Samarra, two central Iraqi cities. Last month, ISIS captured parts of Samarra only to retreat.

The performance of the Iraqi military at Mosul was another source of embarrassment for American officials, who had spent billions of dollars training and equipping the Iraqi military, only to have its soldiers shed their uniforms and flee before the ISIL attackers.

That came as no surprise to Douglas A. Ollivant, who advised both the Obama and the George W. Bush administrations on Iraq after serving two tours of duty there. “The problem is that we trained the Iraqi army to handle a local insurgency. We did not teach them how to deal with a Hezbollah-like, elite paramilitary force,” Ollivant said.

With control of two major Iraqi cities, ISIL could undeniably claim it is a state, with a defined territory that stretches across Syria and Iraq, a justice system and an army, Ollivant said.

Others, however, were not yet ready to grant ISIL so much credit. One U.S. counterterrorism official, who spoke on the condition of anonymity because of the sensitivity of the issue, said ISIL “still has significant weaknesses.”

“It has shown little ability to govern effectively, is generally unpopular, and has no sway outside the Sunni community in either Iraq or Syria,” the official told McClatchy.

The United States has provided a $14 billion foreign military aid package to Iraq that includes F-16 fighter jets, Apache attack helicopters and M-16 rifles. Despite the lack of a U.S. presence in Iraq, U.S. training is ongoing; an exercise is scheduled to take place Tuesday in Jordan between Iraqi and U.S. special forces. At least two F-16s are set to arrive in Iraq by fall, and six Apaches will be leased for training later this year, Pentagon officials said.

U.S. officials were surprised by the speed with which Mosul fell. Until the most recent attack, U.S. officials thought ISIL's grip was limited in geographic scope even as it moved between Iraq and Syria with impunity. But the Mosul attack demonstrated that ISIL “sets the timing. Everybody is reacting to them,” said Jessica Lewis, a research director at the Washington-based Institute for the Study of War.

The Mosul takeover also will have an impact in Syria. On Tuesday, several videos posted online showed ISIL fighters driving U.S.-supplied equipment into Syria.

“They are rearming and refitting their forces with U.S. equipment,” Lewis said.

Two U.S. officials told McClatchy it is unclear how much U.S.-supplied equipment ended up in ISIL hands. So far, they said, there’s no evidence that ISIL has added such significant equipment to its arsenal as Apache helicopters or Hellfire missiles.

But what limits U.S. intervention the most is the divisive political environment in Iraq, where Sunnis and Shiites continue to distrust one another. Many Sunnis is places like Fallujah and Mosul have passively supported ISIL and have accused al-Maliki of fueling sectarian tensions.

When asked whether the sectarian rhetoric of al-Maliki’s Shiite-dominated administration pushes marginalized Sunnis closer to ISIL, Psaki replied that all Iraqi leaders — including al-Maliki — could do more to work toward a unified political vision.